A method is described for the histochemical localization of protein-bound tryptophan, based on the use of nitrous acid, followed by N-(l-naphthyl)ethylenediamine. Of various compounds tested in solution, only indole derivatives and the aromatic amine, p-aminobenzoic acid, gave a color with the reagents. Evidence is presented regarding the probable mechanism of the color reaction and structure of the final colored substance.